[3] Strength testing of these muscles can be measured during a neurological examination to assess function of the spinal accessory nerve.

Injury to the spinal accessory nerve is most commonly caused by medical procedures that involve the head and neck.

In the neck, the accessory nerve crosses the internal jugular vein around the level of the posterior belly of digastric muscle.

As it courses downwards, the nerve pierces through the sternocleidomastoid muscle (approximately 1 cm above Erb's point) while sending it motor branches, then continues down until it reaches the trapezius muscle (entering at the junction of the middle and lower third of the anterior border of the trapezius) to provide motor innervation to its upper part.

[1] The lateral horn of high cervical segments appears to be continuous with the nucleus ambiguus of the medulla oblongata, from which the cranial component of the accessory nerve is derived.

[12] The spinal component of the accessory nerve provides motor control of the sternocleidomastoid and trapezius muscles.

[8] The cranial component of the accessory nerve, on the other hand, provides motor control to the muscles of the soft palate, larynx and pharynx.

As the trapezius and sternocleidomastoid muscles are derived from the pharyngeal arches, some researchers believe the spinal accessory nerve that innervates them must carry specific special visceral efferent (SVE) information.

Others consider the spinal accessory nerve to carry general somatic efferent (GSE) information.

[8] Weakness in both muscles may point to a more general disease process such as amyotrophic lateral sclerosis, Guillain–Barré syndrome or poliomyelitis.

[5] Injury to the nerve can result in shoulder girdle depression, atrophy, abnormal movement, a protruding scapula, and weakened abduction.

[10] Because diagnosis is difficult, electromyogram or nerve conduction studies may be needed to confirm a suspected injury.

[5] Outcomes with surgical treatment appear to be better than conservative management, which entails physiotherapy and pain relief.